Cohesive Soil Erosion: Theory and Practice

Considerable research has been conducted on the erosion of cohesive soils, yet predictive models remain rudimentary at best and few design data are available. This research summarizes the current knowledge on cohesive soil erosion. Cohesive soil erosion is a complex phenomenon, determined not only by soil properties and flow hydraulics, but also by the chemical interaction between the soil pore water and the eroding fluid. While noncohesive soils erode as individual grains, cohesive soils erode as aggregates; thus, interped bonding is also important. The erosion resistance of cohesive soils is further affected by changes in the amount and physical state of soil pore water: significant increases soil erodibility have been correlated to freeze-thaw cycling. Considered a soil property, soil erodibility expresses the rate at which a soil will erode, once erosion starts. Typically, the erosion rate of cohesive soils is predicted using a model relating soil erodibility to a measure of the hydraulic forces on the soil. The most common expression is known as the excess shear stress equation, which states the erosion rate is proportional to the difference between the applied boundary shear stress and the soil critical shear stress. Originally used for noncohesive soils, the critical shear stress is defined as the hydraulic stress at which a soil will erode. For cohesive soils, critical shear stress is difficult to predict accurately; there is no precise definition of critical shear stress as there is rarely a defining the point at which erosion starts. Several researchers have developed empirical relationships between the critical shear stress of cohesive soils and soil properties, but the prediction of fluvial entrainment rates based on soil physical properties has had limited success. This lack of adequate methods to predict soil erodibility and critical shear stress for cohesive soils has led to the development of several field test methods using an impinging jet. Ongoing research is comparing these test methodologies.